An organic electroluminescence device (hereinafter, referred to as an organic EL device occasionally) is a light emitting device having at least an organic light emitting layer disposed between a pair of electrodes. The energy generated by recombination of holes injected from the anode and electrons injected from the cathode in the light emitting layer is taken out in the form of light emission.
The organic EL device is a device spontaneously emitting light. The organic EL device has various advantageous properties such as the great current efficiency of light emission, the light weight and the decreased thickness, and many developments have been made recently. As the drawback of the organic EL device, the luminance of the emitted light decreases as the device is driven, and various improvements are attempted to suppress the decrease in the luminance.
For example, it is disclosed that the decrease in the luminance exhibited by an organic EL device can be suppressed by controlling the concentration of halogen impurities in the organic material used for the organic EL device to a value smaller than 1,000 ppm by mass (for example, refer to Patent Reference 1).
As the process for controlling the concentration of halogen impurities in the aromatic compound used for the organic EL device to a desired value, suitable combinations of purification technologies such as purification by sublimation and purification by recrystallization are disclosed in the above patent reference. Recently, a technology for controlling the content of halogen impurities more effectively is required, and it is necessary that a production technology enabling a further decrease in the content of halogen elements in the material for organic EL devices be developed.
In general, the material for organic EL devices is produced in accordance with a process using a halogenated aromatic compound as the intermediate compound such as the Ullman reaction, the Grignard reaction and the Suzuki coupling reaction. It is known that impurities in the used material greatly affect the properties of organic EL devices such as the decrease in the luminance and the initial current efficiency. In general, the increase in the purity is achieved in accordance with a purification process utilizing the difference in the physical properties of the material such as the purification by sublimation, the purification using columns and the purification by recrystallization.
When the attention is focused on the increase in the purity of the material for organic EL devices, it is important that the contents of halogen compounds which are impurities in the material for organic EL devices be decreased and, in particular, the contents of bromine compounds and iodine compounds having great reactivities be decreased. However, the contents of bromine compounds and iodine compounds cannot be decreased sufficiently in accordance with conventional processes.
[Patent Reference 1] Japanese Patent No. 3290432